The present invention relates generally to rolling mills and more particularly to a method of compensating for friction in a metal rolling mill stand having automatic gage control to control the output gage (thickness) of a metal strip or workpiece.
One well known method of controlling workpiece gage is that which is commonly referred to as the BISRA gagemeter automatic gage control (AGC) system. In this system, the force associated with and generated by the workpiece as it is passed through the stand work rolls is sensed and combined with a signal proportional to roll position to form a signal representative of workpiece thickness which is used in a closed loop system to adjust the gap or opening between the opposed work rolls.
In applications where incoming workpiece hardness and thickness variations are less significant than mill roll irregularities, such as eccentricity or ovalness, the thickness control strategy may be based upon regulation of rolling force on the assumption that constant rolling force will produce uniform output thickness.
In practice these systems have not been as accurate as might be anticipated and one of the primary causes of inaccuracies is friction. As is well known in the art, friction exists between the mill stand housing and the chocks which support the rolls as well as in certain hydraulic elements such as balancing jacks which are used to maintain the roll chocks in position and, where used, the hydraulic roll gap adjustment mechanism. Since both gagemeter and force control systems employ the use of a force feedback signal, it is apparent that any forces seen by the force sensor in addition to those forces produced by reduction of the workpiece will tend to degrade the accuracy of that force signal as a true representation of the actual rolling force. It must be remembered that in all gage control systems the gap between the rolls is repeatedly being changed in an attempt to effect constant output gage as a function of the force feedback signal.
It is also recognized in the art that the frictional forces within the mill stand are additive with respect to the actual workpiece rolling force while the rolls are being moved in a first direction and are subtractive when the rolls are being moved in the opposite direction. This, in effect, produces in a condition generally referred to as hysteresis.
The amount of such hysteresis is a function of the relative centerline positions of work and backup rolls, referred to as roll "offset", the rolling force level, the forward and backward acting workpiece tensions, the backup roll bearing lubrication, the mill window and bearing chock surface condition and lubrication, and the roll balance jack seal condition. Any of these is subject to change, particularly with a change in roll chocks, making prediction of friction forces very difficult.
Because of these frictional forces, without some form of friction compensation, the AGC system is at best inaccurate and, in the worst case, unstable. An example of unstable operation is that resulting from frictional forces in gagemeter systems where force sensor and actuator are on the same side of the roll bite. In this arrangement, the friction force causes the roll position to overshoot in both directions of travel. As a result, it has been the practice of many operators of such systems to detune the control to produce less than complete correction of sensed gage errors. This improves stability but decreases accuracy.
While it is apparent that there is a preferred location for the force sensor with respect to the roll position actuator for a given gage control strategy, the choice may be complicated where both gagemeter and constant force control modes are used. The preferred force sensor location for one mode will produce unstable operation in the other mode, in the presence of substantial friction. Even where the force sensor is in the preferred location, large amounts of friction cause the thickness control system to under correct, resulting in inaccurate, although stable, operation.
As earlier indicated, the friction forces and the hysteresis effect thereof are well known in the art and for a more complete discussion thereof, reference is made to the following two articles:
(a) "Mill modulus variation and hysteresis--Their effect on hot strip mill AGC" by G. E. Wood et al., Iron and Steel Engineer Yearbook, 1977, pages 33 through 39; and,
(b) "Force sensing in rolling mills" by A. Zeltkalns et al., Iron and Steel Engineer Yearbook, 1977, pages 40 through 46.